Solid State Phenomena Vols. 192-193

Abstract: This paper deals with the simulation of two extrusion tests by thixoforming: a non stationary extrusion test and a double-cup extrusion test. The simulations are based on a thermo-mechanical one-phase constitutive law that has been presented in details in previous papers. A campaign of experimental extrusion testing has been conducted on a steel alloy and the comparison between the numerical and experimental results will validate the model under study. A new feature that has been added to the model is also discussed: the introduction of the phase change thermal effects such as the fusion latent heat and the contraction of the material.

Abstract: The problem of an uncompressible power-law fluid has long been the challenge in semi-solid forming area. In this paper, the flow of a power-law fluid film on an unsteady stretching surface is analyzed by the means of Boubaker Polynomials Expansion Scheme (BPES). Analytic solutions were given and compared with the numerical results for some real power-law index and the unsteadiness parameter in wide ranges. The good agreement between them showed BPES could be used effectively to solve the flow of nonlinear power-law fluid in semi-solid state.

Abstract: Semi-solid processing of metallic alloys has been developing over the last 30 years. Millions of components are now manufactured by semi-solid processing. A semi-solid processing so called thixoforming requires reheating the feedstock to a semi-solid state in relatively short time interval with a uniform temperature distribution as well as an optimum liquid fraction. Microstructure, which makes significant impact on processing parameters and quality of the component, changes during the reheating process. The main objective of this study is to establish a quantitative relationship of the microstructure and the induction heating process parameters of the aluminum alloy 319s. This quantitative relationship is employed in the numerical simulation of calculating solid/liquid fraction changes during induction heating process. The simulation results are then successfully applied in aiding optimization of process parameters to make an automobile engine turbocharger compressor wheel, which has very complex geometry.

Abstract: The conventional liquid die casting and rheo-diecasting processes of magnesium matrix composites were studied with the numerical simulation method. The constitutive model of semi-solid Mg2Si/AM60 composite had been established in our prior study. The pressure and the inside defects of the material during rheo-diecasting is realized. The comparison of two forming processes is done. The results indicate that the rheo-diecasting won't lead to interior defects such as gas-hole etc. The simulation results are in accord with the experimental ones.

Abstract: According to semi-solid slurry rheological behavior, an apparent viscosity model of A356 alloy developed based on the Carreau model was established to simulate filling process of rheo-diecasting about automobile shock absorber parts and to compare with conventional liquid filling process. Numerical simulation results showed that the filling process of rheo-diecasting was smooth but difficult to splash, which reduced the tendency of the alloy oxidation and inclusion. Meanwhile, a certain percentage of the primary solid particles precipitated before filling and solidification shrinkage of semi-solid slurry were small. This benefited to reduce or eliminate shrinkage defects of the castings. Compared with conventional liquid die casting process, rheo-diecasting process had unique advantages in reducing the internal defects and improving mechanical properties of castings.

Abstract: An inventive method of flexible thixo-extrusion technology was put forward, in which the stress status on flow front surface was controlled during thixo-forming. A complex part with different dimensions in three axis directions was designed. The velocity of sliding die was controlled by spring. This kind of die construction could make sure that the flow front of the slurry was always in compressive stress status. The velocity field, effective strain fields, effective stress fields were simulated and analyzed. The results showed that the thixo-extruded complex part was produced under compressive forming state, which forecast good mechanical properties in the practical experiment.

Abstract: In this paper, a numerical model for friction welding of thixo-cast materials is developed, which includes a coupling of thermal effect and plastic deformation using a finite element method (FEM). As the constitutive equations for flow behavior of materials for a thixo-cast material are expected to be different from those of conventionally cast material of the same alloy, the necessary material data are experimentally determined from isothermal hot compression tests of the A356 thixocast alloy. The Johnson-Cook model has been employed to represent the flow behavior of the thixocast A356 alloy. The purpose of this FEM analysis is to provide better understanding of the friction welding process of thixo-cast material, and to obtain optimized process parameters before an actual welding is carried out.

Abstract: The key to all semisolid processes is spherical and non-dendritic particles suspended in a liquid matrix. This special kind of microstructure causes the semisolid material to behave like a non-Newtonian fluid dependent on time and shear rate. Semi solid metal processing gives less shrinkage and porosity, non-macro segregation, lower flow stress, good formability and increased die life compared with conventional production methods. Therefore, semi solid processing introduces a good option for massive forming of high temperature alloys. Here, by partial remelting of M2 tool steel alloy under rapid compression test, deformation mechanism of steel alloy and its correlation to rheological properties were investigated. Flow stress for M2 tool steel alloy corresponding to solid fraction above 55% derived at constant strain rate and holding time. In order to investigate the parameter involved in this process, in the second stage of experiments the ram speed and holding time are variable. The analysis of the derived curves shows that the semisolid slurries exhibit a pseudo plastic behavior dependent on stain rate and temperature. The classical power law model used to describe the rheological and thixotropic dependence.

Abstract: The rheological behavior of semi-solid tool steel is investigated using a conventional rheometer under cooling rate and shear rate. The processing of steels in semi-solid state is still at development stage due to the high processing temperature involved. The candidate grades of steel for thixoforming are high speed, high carbon, stainless steel and ductile iron. TheHS6-5-2 tool steel has a wide semi-solid range of over 200 C and the required spheroidal microstructure achieved by partial re-melting from as supplied state without any mechanical agitation, indicated the potential of this grade to be investigated in semi-solid processing. For non-isothermal processing an increase of solid fraction related to solidification due to thermal exchanges is observed. In the first series of experiment after melting the feed stock in an alumina cup, the alloy is sheared continuously by a rotating rod with a constant shear rate under three different cooling rates to the 55% corresponding liquid fraction. In the second series of experiment three different shear rates were applied under a constant cooling rate. The apparent viscosity of the slurry as a function of cooling rate and solid fraction is calculated from the applied torque to the outer cylinder. The results show that the slurry viscosity decreases with increasing shear rate and increases with increasing cooling rate and solid fraction. Additionally for a given solid fraction, shear thinning behavior is observed.

Abstract: The SEED rheocasting process was used to produce semi-solid slurries of hypereutectic Al-17Si-4.5Cu (A390) and Al-15Si-10.5Mg-4Cu alloys respectively. The rheological properties of these alloys in the semi-solid state were characterized at different deformation rates and at variable solid fractions using the parallel plate compression viscometry. The viscosity at different shear rates was calculated using a Newtonian fluid model for increments of deformation during compression. Microstructures of both alloys, after partial solidification and compression, were examined in order to characterize the flow behavior during deformation. The comparison of the viscosity between the A390 and Al-15Si-10.5Mg-4Cu alloys indicated that the high Mg containing alloy possessed a higher viscosity for the range of shear rates and solid fractions investigated. Rheological results showed that at a given solid fraction, the viscosity of both alloys decreased significantly with increasing shear rates, indicating a shear thinning behavior. In addition, a separation of liquid and solid phases was also observed for both alloys in the microstructural study of compressed semi-solid billets.